Over the last several blog posts we’ve been looking at radiation testing. We began by looking at radiation testing in a general sense by discussing the cumulative effects testing (TID) and the single event effect (SEE) testing. We then applied that information to look at radiation testing for high speed ADCs. I’d like to take a step back from the details and specifics related to radiation testing and focus a bit on some of the reasons that advanced testing and screening is performed on space qualified products. In general, there are many types of tests and screenings that are performed on space qualified products to ensure they are indeed worthy of the application. As I’ve pointed out along the way in my last blog series the possibility of replacing a device in a system or repairing a system in some way once it is deployed in space is practically impossible. If not impossible, at the very least it is impractical and extremely costly.
We have focused a lot on radiation over my last several blogs because it is of utmost concern for space applications due to the radiation-rich environment in space. The three naturally occurring sources of radiation in space are the sun, cosmic rays, and trapped radiation.1 Not only is the sun is a source of radiation in space, but it is also a source of tremendous heat. Thus, in addition to radiation, a device employed in space such as a satellite endures wild temperature extremes as well as it orbits the Earth. As it passes between the Earth and the sun the satellite is exposed to the heat from the sun. Once the satellite has the Earth between it and the sun the temperature drastically reduces. Satellites in LEO (Low-Earth Orbit) can see a temperature range from –170o C to +123o C while satellites in higher orbits could see temperatures between –250o C to +300o C.2
The extreme temperatures experience by the satellite must be taken into consideration. Typically, there are temperature control measures employed to reduce the range of temperatures experienced by devices inside the satellite, but these ranges of temperatures are quite large to work with. In most space applications hermetic packages are employed to keep gases and moisture from posing an issue to the sensitive electronics inside the system. Temperature extremes could result in the changing of states for these gases and moisture and could wreak havoc on the electronics. These hermetically sealed packages are screened to ensure they are free of leaks to keep the sensitive electronics inside safe and sound.
Satellite Exposure to Heat and Radiation
The harsh environment in space dictates that we must know how a device will respond being exposed to radiation and drastic temperature changes before we place it into operation in a space application. Specifically, the radiation exposure is the primary reason for all the testing that we’ve been looking at over my last several blogs. As we have discussed the radiation present in space causes different types of upsets to occur. These upsets need to be understood to make sure that a catastrophic failure of a system does not occur once the device is deployed in space.
We looked at SEL (Single Event Latchup) in general and then specifically with high speed ADCs. It is important to understand the SEL performance of a device so that mitigation techniques can be employed if necessary and if system cost allows. If a device latches up it causes a high current condition that could completely drain the batteries supplying the power in a satellite. The latch up could potentially result in a catastrophic destruction of circuitry as well as the system board within the satellite. Different devices have different susceptibility to latch up and different failure mechanisms.
We also discussed SEU (Single Event Upset) and SET (Single Event Transient) effects that occur when devices are exposed to radiation. These are not typically catastrophic like an SEL event could be, but can cause trouble for a system. These must be well understood so that their effects can be mitigated in the system design. For example, it is important to understand how an op amp output SET might affect downstream devices. It would obviously be undesirable for the op amp output transient to push the output voltage near the supply rails and, in doing so, violate an absolute maximum input voltage condition for the downstream device. In our example that we studied in these last few blogs it is important to know what types of transients we see in the output data of a high speed ADC. This may make an impact depending upon the end application. If a transient in the output data is too long it could interfere with proper interpretation of the input signal in the FPGA or downstream ASIC.
Yet another important effect we looked at was SEFI (Single Event Functional Interrupt). Although, there is no permanent damage with these types of events, it is very important to know about these events. If a device is susceptible to continual SEFIs it may not be a very suitable device for space. It is dependent upon the application just how many SEFIs may or may not be tolerable. In some cases, any SEFI event could be of major concern while in other cases, mitigation techniques might be used that allow the system to work around or possibly prevent and/or limit the number of SEFIs.
These stringent sets of tests and screenings take time to complete. The qualification process for components and systems take a good amount of time. It is important to take sufficient time to make sure that they will stand up to the harsh environment of space. I encourage you to look at the references below for more information about the harsh environment of space. I’d also recommend looking at the qualifications and screenings that ADI performs on space grade products. For ADI’s standard space products, you can find more at Standard Space Products Program. For ADI’s RF and microwave products you can see more information on the screening at: RF & Microwave Standard Space Level Products Program.